1. Field of the Invention
This invention relates to a motor control device, and more particularly to a motor control device which drives the reel motor of a magnetic recording and/or reproduction apparatus, the spindle motor of a magnetic disk apparatus or the like to stop rotation of the motor quickly.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an example of the construction of a conventional motor control device of the type described above and particularly shows the construction for putting the motor in reverse torque to quickly stop the motor. Here, motor drive section 2 drives motor 3 in response to drive current control signal C1 and torque application direction control signal C2 supplied thereto from control section 1. Separately, rotational pulse generation section 4 generates rotational pulse signal P1 of a frequency which increases in proportion to the rotation speed of motor 3. Rotational pulse generation section 4 includes, for example, a disk mounted on the rotary shaft of motor 3 and having slits formed equidistant from each other therein so that light passing through the slits is detected to generate rotational pulse signal P1. Rotation drop detection section 5 counts the frequency of rotational pulse signal P1 using clock pulses of a fixed period, compares the count value with a predetermined preset value upon stopping control and sends out rotation drop detection pulse P2 to control section 1 when the speed drops below the predetermined preset value.
Operation of the conventional motor control device is described below.
When rotation stopping signal S1 which indicates stopping of rotation of motor 3 which is rotating in a forward direction is received from the outside, control section 1 sends out torque application direction control signal C2, which indicates reverse torque, to motor drive section 2 while keeping drive current control signal C1 as is to put motor 3 into reverse torque. It is to be noted that rotation stopping signal S1 is a pulse signal which is generated, for example, by depression of a rotation stopping button or the like provided on the apparatus. Thereafter, control section 1 receives rotation drop detection pulse P2 from rotation drop detection section 5 when the rotation speed of motor 3 drops below the predetermined preset value by reverse torque, and instructs motor drive section 2 to stop the application of torque by way of drive current control signal C1. Rotation of motor 3 is stopped quickly in this manner.
It is to be noted that the rotation speed to be detected by rotation stop detection section 5 is set in advance so that, when control section 1 receives rotation drop detection pulse P2 and delivers an instruction to stop application of torque, rotation of motor 3 begins to stop at an optimum low speed, at which, for example, in a disk apparatus, the disk can be removed from a turntable to a tray with no damage.
In the conventional motor control device described above, in order to stop rotation of the motor rotating in forward torque quickly, the motor is put into reverse torque and the time at which the rotation speed of the motor drops to a predetermined speed of rotation is detected from rotational pulses to effect stopping of the motor. However, since the rotation speed of the motor decreases quickly upon stopping and since, as a result, the frequency of the rotational pulses also varies rapidly in an increasing direction, when it is tried to detect a drop of rotation speed, it sometimes occurs that, depending on the relationship in timing between the period of the rotational pulses and the count value of the clock pulses, the motor reaches its stopping point while the predetermined rotation speed is not detected. Since reverse torque still continues, the motor rotates reversely into a runaway condition due to reverse torque, resulting in failure to effect recording or reproduction as planned.